At least 13 people were killed and eight disappeared, including civilians, according to the first reports.

Colombian authorities said Tuesday they had opened an investigation over the possible massacre of 13 people in the south-western province of Nariño, Colombia’s largest coca-growing region, during what some reported would have been a confrontation between FARC dissidents and ELN fighters.

One day before, dissidents of the now dissolved Revolutionary Armed Forces of Colombia were celebrating a birthday in Pueblo Nuevo, in the town of Magüi Payan about 5.30 p.m. local time, when members of the National Liberation Army (ELN) attacked them, according to some inhabitants.

Others reported that the confrontation only involved two groups of FARC dissidents.

At least 13 people were killed and eight disappeared, including civilians, according to the first unofficial reports.

Tumaco's local attorney general Carlos Bastidas said the confrontation was likely linked to illegal mining or over the control of the routes of drug-trafficking.

“Authorities haven't been able to confirm the facts yet and go to the place the clash occurred,” said Jhon Jairo Rodríguez, the secretary of Magüi Payan's government to reporters.

The National Army dispatched a commission to Pueblo Nuevo but has not been able to reach the area yet because of adverse weather conditions, combined with the poor state of the roads.

Last November, the government and the FARC signed a peace accord putting an end to 52 years of armed conflict. Nevertheless, 15 groups of about 700 dissidents in total have refused to accept the terms of the deal, according to a report issued last week by the foundation Paz y Reconciliacion.

The ELN and the government have opened peace negotiations on Feb. 7 in Quito, Ecuador.